This invention relates generally to the switching of transistors and particularly to the switching of power transistors.
When power transistors are used in switched mode power electronic circuits such as regulators and inverters it is essential, for satisfactory operation, to ensure that the collector current-voltage excursions of the transistors, during switching, are kept well within the safe operating areas of the transistors.
These safe operating parameters are generally well known and many techniques have been proposed for ensuring that they are complied with during transistor switching. Thus various types of snubber circuits are known for achieving the necessary variation in load conditions between transistor turn-on and turn-off.
It is also known to make use of a resonance commutated circuit, for example in half or full-bridge invertors, to generate collector voltage variations so that selected transistors may be switched at substantially zero collector-emitter voltages.
Yet another arrangement which has been proposed is the so-called collector catcher circuit wherein the base drive source is connected to the base and collector of the transistor by means of a plurality of diodes.
In many respects the techniques which have been referred to, or variations thereof, are highly satisfactory but such techniques are often implemented at the cost of complexity.
It is known that a base drive for a transistor operating in a resonance commutated system should possess the following properties:
(1) during the on-state the base current should maintain V.sub.ce (collector-emitter voltage) as low as possible, but without driving the transistor into saturation;
(2) at turn-off the base should be driven negative as fast as possible to ensure efficient turn-off;
(3) the transistor should be prevented from being turned on unless V.sub.ce is below a predetermined value, and
(4) the transistor should be turned off should V.sub.ce rise above some predetermined value.
German Auslegeschrift No. 1 257 842 discloses a circuit which achieves the first of these objectives. The remaining objectives are, however, not addressed.